Multiplane information storage system and record carrier for use in such a system

ABSTRACT

An information storage system is described which comprises a reading device (6) and an optical record carrier (5) having at least two information planes (1, 2, 3). The radiation from the record carrier is converted in a detection system (15) into a detection signal (16) which is applied to a detection circuit (17). In order that this circuit can derive the read information from the detection signal in a reliable manner, the interference signals generated by the information planes which are not to be read should comply with a requirement, referred to as the interference requirement, which is characteristic of the detection circuit. Values for the parameters of the record carrier, such as the thickness of the layers between the information planes and the reflection and transmission coefficients of the information planes then follow from this interference requirement.

This is a continuation of application Ser. No. 08/487,615, filed Jun. 7,1995, now U.S. Pat. No. 5,511,057 which is a continuation in part ofapplication Ser. No. 08/175,331, filed Dec. 29, 1993, now abandoned.

BACKGROUND OF THE INVENTION

The invention relates to an information storage system comprising anoptical record carrier having at least two information planes, and areading device for scanning the information planes from one side of therecord carrier, said device comprising a first optical system forforming a radiation spot on an information plane to be read, a secondoptical system for passing radiation from the record carrier to aradiation-sensitive detection system which converts the radiation intoan electric detection signal, and a detection circuit electricallyconnected to the detection system for converting the detection signalinto an information signal. The invention also relates to an opticalrecord carrier for use in such a multiplane information storage system.

In the aim for increasing the information density in record carriershaving a plurality of information planes, the distance between theinformation planes is to be minimized, while separate reading of theinformation stored in each information plane should remain possible. Afirst step of storing more information on a record carrier is thedouble-sided record carrier i.e. a record carrier in which aninformation plane is present at each side. The two planes are read fromdifferent sides of the record carrier. A subsequent step is to providetwo or more information planes on a record carrier, which planes can beread from one side. By securing two of these multiplane record carriersvia one side, the information contents can be further increased by afactor of two.

An information storage system of the type described in the openingparagraph is known from U.S. Pat. No. 3,855,426. The reading device ofthe storage system focuses a radiation beam on one of the informationplanes of the record carrier. The information is stored in informationareas, or marks in the information planes. The radiation passed by therecord carrier is modulated by the marks and is received by a lens whichforms an image of the area of the information plane to be currently readon a radiation detection system. The detection system converts themodulation of the incident radiation into an information signal. If theradiation beam is focused on an information plane, it will alsoilluminate an area on a different, higher-or lower information plane.This area or parasitic area whose illumination is unwanted should be solarge and contain so many marks that the influence of the separate markson the desired reading signal is averaged. The average influence of themarks in the parasitic area on the information signal will then be nomore than a reduction of the modulation depth of the information signalgenerated by the interaction of the radiation beam with the marks in theplane to be read. To achieve this effect, the distance between theinformation planes in the known record carrier is taken to be largerthan 10 μm, dependent on the numerical aperture of the objective system.

Said U.S. Pat. No. 3,855,426 does not state the requirements to besatisfied by the reading device and the record carrier in order that thereading device can derive the information stored in an information planeto be read with sufficient reliability from the radiation originatingfrom the record carrier. Knowledge of these requirements is of greatimportance due to the trend of increasing the information density inrecord carriers and the related wish of positioning the informationplanes closer together so that there is a greater risk of crosstalkbetween the information planes while the reading device becomes morecritical.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an informationstorage system in which a maximum information density is achieved bymutual adaptation of the requirements imposed on the reading device andthe record carrier, which storage system is based on newly gainedinsights into the importance and influence of given system parameters onthe information signal which is ultimately obtained.

The information storage system according to the invention ischaracterized in that the distances between the information planes andthe optical properties of the information planes are such that theinformation storage system complies with the interference requirement,i.e. the ratio between the sum of interference signals in the detectionsignal generated by the information planes not to be read and a readsignal in the detection signal generated by the information plane to beread is smaller than an interference ratio Q determined by the detectioncircuit.

It has been found that the critical parameter of the reading device isthe maximum interference ratio, i.e. the ratio between the strength ofthe interference signals and the desired read signal in the detectionsignal, at which ratio the detection circuit is still just capable ofderiving the information from the detection signal with a reliabilitywhich is sufficient for a particular read action of the system. At agiven strength of the desired read signal the maximum interference ratioimposes an upper limit on the allowed strength of the interferencesignals. If the interference signals exceed the upper limit, thegeneration of the information signal from the detection signal will beinfluenced by the interference signals and the detection circuit will nolonger supply reliable information. The interference signals may becaused, inter alia, by fluctuations in the power of the radiation beamsupplied by the radiation source, scattered light in the optical systemof the reading device, roughness of the information plane to be read.The information planes not to be read also produce interference signalsbecause they also send radiation from the read beam to the detectionsystem. When the reading device is designed, the total budget for theinterference signals is distributed over the various sources ofinterference. A part of the budget should therefore be allocated to theinterference signals originating from the information planes which arenot to be read. This part of the budget corresponds to the interferenceratio Q. If the information storage system complies with theinterference requirement, i.e. if the interference ratio being thequotient of the interference signals caused by the information planesnot to be read and the read signal is smaller than Q, the informationwill be read in a reliable manner, assuming that the strength ofinterference signals from other sources remains within the part of thebudget allocated to these signals. By making use of the novel notion ofinterference requirement, it will be possible to attune the readingdevice and the record carrier to each other in an optimum way. When therecord carrier is designed, the parameters such as the distance betweenthe information planes, the reflection and transmission of theinformation planes and the extent to which the marks of the separateinformation planes modulate the incident radiation should be chosen insuch a way that the interference requirement is satisfied. The maximuminformation density in the record carrier in the direction perpendicularto this carrier and in the plane of the information plane is achieved ifthe interference ratio is equal to Q.

A special embodiment of the information storage system according to theinvention is characterized in that the interference requirement isdefined by ##EQU1## in which the summation over j is over allinformation planes not being the information plane i to be read, and thesummation over f is over the frequencies present in the signal receivedfrom an information plane, while E_(j) is the optical power of theradiation from information plane j, m_(j) (f) is the modulation factorat frequency f of the information plane j, in which m_(j) (f) for aninformation plane not to be read is determined with the radiation beamfocused on plane j, and in which further MTF(f,d_(j) /n) is themodulation transfer function at the frequency f of the radiationoriginating from the information plane j to the detection signal, d_(j)is the distance between the information plane j and the informationplane i to be read, and n is the refractive index of the medium betweenthe information planes i and j.

A special embodiment of the information storage system according to theinvention, in which the correct functioning of the detection circuit isnot dependent on the sum of the amplitudes of the electricalinterferences signals but on the sum of the powers of the interferencesignals, is characterized in that the interference requirement isdefined by ##EQU2##

For detection circuits which are sensitive to the DC component of thedetection signal, the interference signals of zero frequency should betaken into account. In such detection circuits the sensitivity to DCinterference signals may be different than for AC interference signalsso that there is both an AC value and a DC value for Q. In the design ofthe record carrier two interference requirements should therefore betaken into account, i.e. the interference requirement for the ACinterference signals with the AC value for Q and the interferencerequirement for the DC interference signals with f=0, m_(i) =m_(j) =1and the DC value for Q.

If the distance between the information planes is small as compared withthe depth of field of the objective system, the amplitude addition ofradiation of neighbouring information planes should be taken intoaccount when the value of E_(j) is determined. At larger distances theradiation of neighbouring information planes should be added in power.

If a record carrier has three information planes 1, 2 and 3, in whichplane 1 is closest to the objective system and plane 3 is read inreflection, a part of the radiation focused on plane 3 will be reflectedfrom plane 2. At equal distances between the three planes, thisreflected radiation will focus on plane 1 and after reflection fromplane 1 and reflection again from plane 2, it will be passed towards thedetection system. Due to these reflections from plane 2 an unexpectedlylarge interference signal is generated by plane 1. An informationstorage system according to the invention, which can be optimized forsuch a large interference signal, is characterized in that the recordcarrier has at least three information planes and in that the sum insaid interference requirement includes imaginary information planes eachconstituted by mirroring an information plane with respect to anotherinformation plane. The interference requirement can be satisfied by acorrect choice of distances between the planes and/or reflectioncoefficients of the planes.

A special embodiment of the information storage system according to theinvention is characterized in that a first information plane has aninformation structure which is optimally read in a first read mode, andat least a second information plane has an information structure whichis optimally read in a second read mode, in that the reading device hasthe disposal of both read modes and in that the parameters E_(j), m_(j)and MTF(f,d_(j) /n) in the interference requirement for an informationplane which is not to be read have those values which are associatedwith the read mode in which the information plane i to be read is read.

Let it be assumed that the record carrier has two information planes inwhich the information in the first information plane is stored in afirst information structure, for example in magnetic domains, whichstructure must be read in a so-called differential mode, and in whichthe information in the second information plane is stored in a secondinformation structure, for example in pits, which structure must be readin a so-called central aperture mode. When the first information planeis being read, the detection signal is generated by the detection systemoperating in the differential mode. To determine the interference in thegenerated read signal, the value of the interference signals of thesecond information plane must also be determined by means of thedetection system in the differential mode instead of in the centralaperture mode.

The different read modi can be realised in several manners. A firstembodiment of the reading device of the information storage system usingtwo read modi is characterized in that the radiation source is adaptedto supply a radiation beam having a first wavelength in the first readmode and a radiation beam having a second wavelength in the second readmode. By giving the information planes different properties for the twowavelengths, it is possible to further reduce the interference signals.

A second embodiment of the reading device of the information storagesystem using two read modi is characterized in that the detection systemhas the disposal of two detection modi for detecting, in the first andthe second read mode, the radiation from the record carrier in the firstand the second detection mode, respectively. Detection in several modiprovides the possibility of efficiently converting the radiation from aninformation plane to be read into a read signal, and of suppressing theinterference signals which are generated by the radiation from otherinformation planes in which the information is stored in a differentmanner.

A special embodiment of the information storage system according to theinvention, in which the information planes are read in reflection, ischaracterized in that the radiation intensity reflection coefficients ofthe successive information planes are defined by ##EQU3## with anincreasing distance of the objective system.

Each information plane to be read then supplies an equal amount ofradiation to the detection system so that the detection circuit of eachinformation plane receives an equally large detection signal.Consequently, varying values of the detection signal need not be takeninto account in the detection circuit, which simplifies the constructionof this circuit.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings

FIG. 1 shows an information storage system,

FIG. 2 shows the variation of a detection signal as a function of time,

FIG. 3A shows an information storage system operating at twowavelengths,

FIG. 3B shows an associated record carrier having two information planesin one layer,

FIG. 3C shows an associated record carrier having two separateinformation planes,

FIG. 3D is a magnification of the two information planes of FIG. 3C,

FIG. 4A shows an information storage system in which marks in the formof pits and domains are used,

FIG. 4B is a cross-section of a record carrier having two informationplanes used in this system,

FIG. 5A shows a record carrier having three information planes,

FIG. 5B shows a record carrier having intermediate layers with differentthicknesses,

FIG. 6 shows the variation of the modulation transfer function as afunction of the normalized distance between the focal plane and theinformation plane upon reading in reflection,

FIG. 7A shows an information storage system in which the signals areread in transmission, and

FIG. 7B is a cross-section of the record carrier of FIG. 7A.

Tables 1A, B, C contain a copy of numbered mathematical formulae of theFigure description.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows diagrammatically an information storage system comprising arecord carrier 5 and a reading device 6. The record carrier, part ofwhich is shown in a cross-section in FIG. 1, comprises a transparentsubstrate 7 which is provided with three information planes 1, 2 and 3separated by intermediate layers 8 and 9. An information plane maycomprise a single interface between two media with different opticalproperties, or one or more layers between two media. The information isrecorded in the information planes in marks which are not shown in theFigure. The marks may be arranged in parallel tracks. The marks may be,for example pits or domains magnetized in a given direction, or areashaving a reflection coefficient which deviates from their surroundings.The information stored in the information planes is read by means of thereading device 6 in which a radiation beam 10 is generated by aradiation source 11, for example a semiconductor laser. The radiationbeam travels from the source to the information plane via a firstoptical system. The optical system comprises a beam splitter 12, forexample a partially reflecting mirror, a collimator lens 13 and anobjective system 14 shown in the Figure as a single lens which focusesthe beam to a spot 18 on the information plane 1, 2 or 3 to be read. Thespot 18 of the reading device may be located on any desired informationplane, for example, by moving the objective system 14 along the opticalaxis, as is denoted by means of the arrow 19 in FIG. 1. By moving thespot 18 and the record carrier with respect to each other in a planeparallel to the record carrier, any information plane can be scanned.The radiation reflected by the relevant information plane is thenmodulated by the marks in this information plane. This radiation isguided to a detection system 15 via a second optical system, comprisingthe objective lens 14, the collimator lens 13 and the beam splitter 12.The detection system converts the incident radiation into an electricdetection signal S_(d) whose modulation is related to the modulation ofthe radiation beam. A detection circuit 17 derives an information signalS_(i) from the detection signal, which information signal represents theinformation read.

In many cases the incoming detection signal is compared with a givendetection level in the detection circuit in order to convert the analogdetection signal to a digital information signal. FIG. 2 shows by way ofexample a particular detection signal S_(d) as a function of time t. Thedetection level D is indicated by means of a broken line. The detectioncircuit 17 reconstructs the information signal S_(i) by using theinstants t₁, t₂, t₃, etc., at which the value of the detection signal isequally large as that of the detection level.

In addition to the read signal originating from the currently readinformation plane, the detection signal also comprises interferencesignals, which may give rise to shifts of said instants. If the sum ofall interference signals exceed a given strength or power, the shiftswill lead to a decrease of the quality of the information signal S_(i).If the stored information is a video program, said shifts will lead to anoticeable deterioration of the quality of the picture; in the case ofinformation in the form of digital data the shifts lead to errors in theinformation signal whose allowed number is dependent on the use of theinformation storage system. Consequently, for each reading device anupper limit is to be imposed on the interference signals. If the sum ofthe interference signals in the detection signal S_(d) remains below thelimit value, the detection circuit can supply a reliable informationsignal S_(i). The interference signals in the reading device areproduced, inter alia, by fluctuations in the power of the radiation beam10 supplied by the radiation source 11, scattered light produced in theoptical system of the reading device, and roughness of the informationplane to be read. The designer of the information storage system willallocate a part of the maximum allowed interference signal budget toeach possible interference signal source. A part of the budget should beallocated to the interference signals which are produced because theinformation planes located proximate to the information plane to be readalso direct radiation towards the detection system 15. The maximum valueof these interference signals is represented for each multiplaneinformation storage system by a new parameter for the record carrier,the value of which parameter is determined by the detection circuit 17.This parameter is the interference ratio Q, i.e. the ratio of themaximum interference signal from all information planes not to be readcurrently and referred to as interference planes, and the read signal ofthe information plane to be read currently. The ratio of the maximumelectric power in the interference signals and the electric power in theread signal is equal to Q².

For a correct operation of the detection circuit it is necessary thatthe record carrier satisfies the following interference requirement: theratio between the interference signal generated by the interferenceplanes and the read signal should be smaller than Q. The value of Q isdetermined, inter alia, by the way of generating the detection signal inthe detection system 15, the possible presence of correction filters forimproving the detection signal, the way in which the information signalin the detection circuit 17 is derived from the detection signal, andthe requirements which are imposed on the information signal. The safetymargins within an allocated part of the maximum allowed interferencesignal budget should also be taken into account. Some systems may haveseveral values for Q, one value for each type of information. One typeof information is user data with essentially a random character. Anothertype is address information used for labelling region on an informationplane; such information has comparable contents in neighbouring tracksand in neighbouring information planes. A third type of information istracking information used for guiding the spot 18 along the tracks. Thetype of information giving the strictest interference requirementdetermines the design parameters of the record carrier. Althoughexamples below deal with random information, the interferencerequirements for other types of information can be derived in ananalogous way.

If the information is stored in the carrier in a digital form, it ispossible to store error-correction information with it. Theerror-correction information can be used in the detection circuit tocorrect errors in the information signal. A larger amount oferror-correction information allows correction of larger errors,lowering the quality requirements of the detection signal. Hence, adetection circuit with a great error-correction capability will have arelatively large value of Q, as it can cope with large interferencesignals. The value of Q will hereinafter be considered as a factor fixedby the reading device. The interference signals referred to hereinafterare interference signals generated by interference planes, unlessotherwise stated.

The value of Q should be taken from the design of the informationstorage system. In case this value is not known for a system, anapproximate value can be obtained in the following way. The modulationof the detection signal S_(d) must be measured when the reading devicereads an optical record carrier with only one information plane. Thequality of the record carrier should be the minimum quality allowed bythe specification of the system, such that the part of the budgetallocated to the interference signals not related to interference planesis fully used. The remaining part of the budget is then for theinterference signals generated by the interference planes of multiplanerecord carriers. The magnitude of this part can be determined by addinga signal with a controlled modulation to the detection signal S_(d).This modulation must be increased until the system reaches a minimumperformance level. The ratio of the controlled modulation and themodulation due to the single-plane record carrier is now an approximatevalue for the parameter Q.

When the radiation beam is focused on an information plane, itilluminates an area on this plane, which area is referred to as the spot18. The sizes of the marks and the spot are generally, but notnecessarily of the same order of magnitude. The presence of the marksinfluences the beams transmitted and reflected by the information plane.If the radiation beam and the record carrier move with respect to eachother, so that the spot scans the information plane, said beams will bemodulated by the marks. In the system shown in FIG. 1 the reflected beamis passed towards the detection system of the reading device. In acomparable manner, the transmitted beam can be detected by means of adetection system arranged above the record carrier. It will hereinafterbe assumed by way of example that the information is stored in the formof marks and intermediate lands, arranged in tracks scanned by the spot.The direction parallel to the tracks is referred to as the tangentialdirection and the direction perpendicular to the tracks and in theinformation plane is referred to as the radial direction. Although thisnomenclature refers to a disc-shaped record carrier, the invention isnot limited to this, but includes record carriers of all shapes, such asoptical cards. Upon reflection of the radiation beam on the informationplane, a part of the incident radiation will be diffracted by the marks.Diffraction beams of the first and higher orders are produced, whichbeams leave the information plane in other directions than thenon-diffracted zero-order beam. The angle between the higher-orderdiffraction beams in the tangential direction and the zero-order beam isa measure of the local period in this direction of the structure ofsuccessive marks and lands. The power of a higher-order diffraction beamin a given direction is determined by the number of marks present in theread track with a period between them associated with the deflectionangle of the beam, and by the size of these marks and the contrastbetween the marks and their surroundings. In other words, thediffraction beams in the tangential direction constitute aone-dimensional Fourier transform of the marks in the track andrepresent the contents of the stored information in terms of spatialfrequencies. The higher-order diffraction beams cannot only bedistinguished from the zero-order beam in their direction of propagationand power, but also in their phase or state of polarization, dependenton the properties of the marks.

The zero-order and higher-order diffraction beams are passed through anoptical system comprising the objective system 14, the collimator lens13 and the beam splitter 12 to the detection system 15. In aninformation structure comprising marks having a reflection coefficientdifferent from their surroundings, the radiation portions of thezero-order and higher-order beams incident on the detection system willinterfere with each other. The DC value of the amplitude of thegenerated electric detection signal is proportional to the power in thezero-order beam, while generally its AC value is proportional in a goodapproximation to the product of the amplitude of the zeroorder beam andthe amplitude of the higher-order diffraction beams, as is known, interalia from the book "Principles of Optical Disc Systems" by G. Bouwhuiset al (Hilger, 1985), chapter 2. This chapter describes how thedetection signal can be calculated from the power in the diffractionbeams. If the optical power, i.e. the square value of the amplitude, ofthe zero-order beam originating from information plane i is representedby E_(i), the DC value of the amplitude of the associated detectionsignal is proportional to E_(i). The AC value of the amplitude isproportional to E_(i) m_(i) (f) in which the modulation factor m_(i) isproportional to the ratio of the amplitude of the higher-orderdiffraction beams and the amplitude of the zero-order beam. Theparameter f is the frequency of the detection signal. It indicates thefrequency dependence of the modulation factor, i.e. the dependence onthe information contents of the track which has been read. A part of theradiation in the higher-order beams will not be received by thedetection system, because the large deflection angle of these beams willcause a part of them to fall outside the objective system and to getlost. The larger the angle between the first-order diffraction beam andthe zero-order beam, the larger the part falling outside the objectivesystem. The resultant loss of power of the detection signal is expressedin the modulation transfer function MTF(f). Due to the movement of themarks in the track with respect to the spot, the spatial frequency ofthe tracks is converted into a temporal frequency of the detectionsignal. The read signal S_(r), i.e. the part of the detection signalS_(d) produced when the marks in the track to be read are scanned cannow be written as: ##EQU4## in which C is a detection constant for theconversion from optical radiation to electric signal, and m_(i) (0)=1.The summation is over all frequencies which occur in the track to beread. If the frequencies have no discrete but a continuous distribution,the summation in equation (1) should be replaced by an integration.Although the reasoning leading to equation (1) is based on aninformation structure having marks of different reflection coefficients,the read signal of each information structure can be written in the formof equation (1). The modulation factor m_(i) is dependent on theproperties of the information structure, hence on the properties of thediffraction beams such as the relative power of the beams, mutual phaserelation and state of polarization, and on the way in which theradiation in the detection system 15 is converted into an electricdetection signal. The modulation transfer function MTF is dependent onthe properties of the optical system with which the radiation is focusedon the information plane, and on the optical system with which theradiation from the information plane is passed towards the detectionsystem, i.e. of the collimator lens 13 and the objective system 14 inthe reading device shown in FIG. 1.

Information planes which are not to be read currently and which arelocated in the radiation path of the scanning beam also direct radiationtowards the detection system and will thus each provide a contributionin the form of an interference signal to the detection signal S_(d).Each of these interference signals can be expressed in an analogousmanner as has been done in equation (1) for the read signal S_(r). Thesum of these interference signals from the information planes which arenot to be read gives the total interference signal. The interferencerequirement for reading information plane i can now be written as:##EQU5## In this formula m_(j) (f) is the modulation factor of aninformation plane j if the radiation beam is focused on plane j, whileE_(j) is the power of the radiation from the information plane j if theradiation beam is focused on the information plane i to be read. Themodulation transfer function MTF for an information plane not to beread, which is also called an interference plane, will often bedifferent from the modulation transfer function of the information planeto be read. This is particularly the case if the spot is not located onthe interference plane. With an increasing defocusing of theinterference plane, the value of the transfer function of this planedecreases rapidly. This defocusing dependence is indicated by means ofthe parameter d_(j) /n in equation (1), in which d_(j) is the distancebetween an interference plane j and the information plane i to be readon which the radiation beam is focused, and n is the refractive index ofthe intermediate layer between the planes i and j. If there are layersof different refractive indices between the information planes, thevalue of d_(j) /n should be determined for these layers together.

In the enumerator of the interference requirement (2a) the opticalpowers of the radiation of the different interference planes are addedtogether. Since an optical power is converted into an electric amplitudein the detection system 15, the addition of the optical powers inequation (2a) will mean that the electric amplitudes of the interferencesignals are added together. The interference requirement (2a) thereforeapplies to those detection circuits 17 whose correct operation dependson the instantaneous amplitude of the interference signal. However, ifthe correct operation of a detection circuit depends on the averagepower in the interference signal, the electric powers instead of theamplitudes of the interference signals must be added together in theenumerator of the interference requirement. For such a detection circuitthe interference requirement then is: ##EQU6## In equations (2a) and(2b) the magnitude of the MTF should be used, as the MTF can becomenegative or complex when there is defocusing or aberrations.

The read and interference signals generally comprise an AC component anda DC component. Both components are incorporated in the interferencerequirement by taking the summations over f in equation (2a) or (2b)from zero. The DC component may result in a shift of the level of thedetection signal S_(d) in FIG. 2 with respect to the detection level D,which gives rise to a shift of the detected instants where the detectionsignal has the value of the detection level. The result is a decrease inreliability of the information signal. In some detection circuits thesensitivity to the DC component differs from the sensitivity to the ACcomponent. If this is the case, the interference requirement should besplit up into two requirements. The AC interference requirement is thengiven by equation (2a) or (2b) with the summations over f starting at afrequency of more than 0 and with Q being replaced by Q_(AC). The DCinterference requirement is then given by: ##EQU7## in which Q_(DC) isthe DC interference ratio of the detection circuit. The modulationfactors m_(j) and m_(i) for the zero frequency are equal to 1, as is themodulation transfer function MTF(0,0).

To comply with the interference requirement, it will be attempted tomaximally reduce the interference signals generated by informationplanes which are not to be read. On the other hand a maximum possibleread signal is wanted during reading of an information plane to ensuresatisfactory operation of the detection circuit. A larger read signalhas the advantage that there will be relatively less noise in the readsignal so that the information signal can more easily be derived fromthe detection signal. This provides the possibility of increasing thequantity of information read per unit of time or increasing theinformation density of the information plane. However, a decrease of theinterference signal generated by an information plane often also leadsto a decrease of the read signal when this information plane is beingread. If the interference signal of an information plane is, for exampledecreased by a reduction of the modulation factor of the informationplane, this will also lead to a smaller modulation of the read signal ofthe information plane when said information plane is being read. Theparameters of the record carrier should therefore be chosen to be suchthat the interference requirement is just complied with. This means thateach information plane to be read furnishes a maximal read signal, whilethe interference signals of the information planes not to be read havethe maximally allowed value.

In principle, the reduction of the interference signals so as to complywith the interference requirement can be achieved by adapting eachparameter in equation (2a) or (2b). For example, it is possible tosuppress the quantity of detected radiation E_(j) of an interferenceplane; alternatively, the modulation factor m_(j) (f) of an interferenceplane can be reduced: moreover, the modulation transfer functionMTF(f,d_(j) /n) of an interference plane can be reduced. Theinterference requirement must be determined for reading each informationplane in the record carrier. The parameter values for which each ofthese interference requirements is satisfied determine the structure ofthe record carrier. Examples of the ways in which said possibilities canbe implemented in an information storage system will be givenhereinafter. In most examples the influence of one interference plane onreading an information plane will be dealt with. The interferencesignals of possible other interference planes can be determined in ananalogous manner and incorporated in the interference requirements inaccordance with equation (2a) or (2b).

A first possibility of reducing the interference signals is to make useof different read modes for the information planes. To this end theinformation planes may have different wavelength sensitivities. Areading device based on this principle is known, inter alia from GermanPatent Application no. 37 41 910. FIG. 3A shows diagrammatically such adevice which operates at two wavelengths. Two radiation sources 20 and21, for example semiconductor lasers, generate radiation at wavelengthsλ₁ and λ₂, respectively. The radiation beams of the two lasers arecombined by means of a neutral mirror 22 and a dichroic mirror 23. Theradiation beams are reflected by a mirror 31 to the objective system 14which focuses the beams on an information plane within the recordcarrier 5. By rotating the record carrier by means of a motor 24, theradiation beams can scan a track of the record carrier. The radiationreflected by the record carrier is reflected by a beam splitter 25 fromthe on-going beam to a second dichroic mirror 26 which passes radiationhaving wavelength λ₁ to a detection system 27 and radiation havingwavelength λ₂, to a detection system 28. The detection signal of eachdetection system is passed to a detection circuit 29, 30 which generatesan information signal from the detection signal. The reading device ofFIG. 3A provides the possibility of reading different information planessimultaneously. If simultaneous reading is not required, only onedetection arm will be needed and the dichroic mirror 26, the detectionsystem 28 and the detection circuit 30 can be dispensed with.

An embodiment of a record carrier 5 having two non-spatially separatedinformation planes for use with the reading device of FIG. 3A is shownin FIG. 3B. The information planes constitute a single thin layer 1 on asubstrate 7. A mark having a reflection coefficient different from itssurroundings, mainly for the wavelength λ₁, can be written in the layerby means of a high-energetic radiation beam 32 having wavelength λ₁.This writing operation can be performed by means of a method known asthe "spectral-hole burning" method. If the marks thus written are readwith a radiation of wavelength λ₁, a good read signal is also obtained,while a weak read signal is obtained when these marks are read withradiation of wavelength λ₂. Conversely, a weak read signal is obtainedwhen marks written with radiation of wavelength λ₂ are read withradiation of wavelength λ₁, whereas a good read signal is obtained whenthese marks are read with radiation of wavelength λ₂. The requirementswith which the marks should comply follow from the interferencerequirement, i.e. equation (2a). On reading at a single wavelength, forexample λ₁, the information plane to be read coincides with theinformation plane not to be read, hence E₁ =E₂, and d₂ =0. Themodulation transfer function for radiation of wavelength λ₁ coming fromthe two information planes is equal for the same reason. Finally, if thetwo information planes comprise information of approximately the samefrequency spectrum, the interference requirement for reading atwavelength λ₁ is reduced to: ##EQU8## The value of the modulation factorm_(j) is dependent on the dimensions of the marks and on the differencein amplitude reflection of the mark and its surroundings, the amplitudereflection being equal to the square root of the intensity reflection.If the marks written at each of the two wavelengths have the samedimensions, the interference requirement (4) can be written as ##EQU9##in which R is the intensity reflection coefficient of the informationplane outside the marks, R₁ is the intensity reflection coefficient of amark written with radiation of wavelength λ₁ and R₂ is the intensityreflection coefficient of a mark written with radiation of wavelengthλ₂. In this case it has been assumed for the sake of simplicity that theintensity reflection coefficient R is equal for the two wavelengths. Ifthe interference ratio Q for interference signals coming frominformation planes not to be read is taken to be 0.03, R is equal to0.30 and R₁ measured at wavelength λ₁ is 0.05, then the reflectioncoefficient R₂ also measured at wavelength λ₁ should deviate by lessthan 0.01 from 0.30, i.e. the value of R. The same consideration appliesto reading with radiation of wavelength λ₂. The marks not to be readshould thus have almost the same reflection coefficient as theirsurroundings. The layer 1 should have such a chemical composition thatthe spectral sensitivity of the reflection coefficient satisfies theserequirements. If the reading device simultaneously reads the twoinformation planes by focusing radiation of the two wavelengths on theinformation plane, the radiation of the unwanted wavelength which, viathe non-ideal dichroic mirror 26, still reaches a detection system notintended for this purpose should be incorporated as an extrainterference signal in the interference requirement. The quality of thedichroic mirror is thus also decisive for the quality of the informationsignals generated during simultaneous reading.

FIG. 3C shows another embodiment of a record carrier 5 for use with thereading device of FIG. 3A. This embodiment has two separate informationplanes, each of which to be read by radiation of a different wavelength.It has the advantage that the materials constituting the two informationplanes can be optimized independently of each other and thus provide agreater freedom of design. The information planes 1 and 2 are separatedby an intermediate layer 8 which may have a thickness of zero so thatthe two information planes are situated directly on each other. Such arecord carrier with an intermediate layer thickness of 2 μm is known inprinciple from FIG. 1 of said German Patent Application no. 37 41 910.However, this Patent Application does not state the value of thewavelength sensitivity of the reflections in relation to the propertiesof the detection circuit in the reading device so as to satisfactorilyread the information in this record carrier. The interferencerequirement in accordance with the present invention, given in equation(2a), furnishes the required relations between the parameters which arenecessary for the design of the information storage system. To obtain amaximum possible information density, the distance between theinformation planes should be chosen to be as small as possible,preferably of the order of or smaller than the wavelength of theradiation beam. In spite of such a small intermediate distance, theinterference requirement can be satisfied by a correct choice of thewavelength dependence of the reflections.

The consequences of the interference requirement for the record carriershown in FIG. 3C will now be explained with reference to FIG. 3D inwhich the reflection of a ray 33 of the read beam on the informationplanes 1 and 2 is shown. In the case to be considered information plane2 is read with radiation of wavelength λ₂ and the permissibleinterference caused by information plane 1 should be determined. Theelectric field of the incoming ray 33 in FIG. 3D has an amplitude a. Theamplitudes of the rays reflected by the information planes 1 and 2,represented by rays 34 and 35, are a√R₁ and aT₁ √R₂ , respectively, inwhich R₁ and T₁, are the intensity reflection and transmissioncoefficients of information plane 1 and R₂ is the intensity reflectioncoefficient of information plane 2, while all coefficients apply towavelength λ₂.

The power of the read signal generated by the information plane 2 isdetermined, inter alia, by the power of the zero-order and higher-orderdiffraction beams reflected by the information plane. Since theamplitude of the beam incident on the information plane 2 is equal toa√T₁ , the amplitude of the zero-order diffraction beam formed byinformation plane 2 is equal to a√T₁ R₂ and the amplitude of thehigher-order diffraction beams is equal to a√T₁ R₂ m₂. In this case ithas been assumed that the power in the higher-order beams is small ascompared with the power in the reflected and transmitted beams. If theinformation plane 1 were only to transmit and not reflect radiation, thezero-order and higher-order beams would interfere on the detector aftertransmission by the information plane 1 and would yield a read signalproportional to a² m₂ T₁ ² R₂. However, the beam 34 reflected by theinformation plane 1 should be added to the zero-order beam coming fromthe information plane 2. If the distance between the information planesis much smaller than λ₂, the amplitudes of the zero-order beam 34 andthe beam 35 may be added together to give constructive interference. Thesum of the zero-order beam of the information plane 2 and the beam 34now interferes on the detector with the higher-order diffraction beamsformed by the information plane 2. This yields the following read signalS_(r) at the output of the detector: ##EQU10## This expression is nowequal to E_(i) m_(i) with i=2 in the denominator of the interferencerequirement given in equation (2a).

When the information plane 2 is being read, the information plane 1 isan interference plane giving an interference signal. In the embodimentof the record carrier shown in FIG. 3D, the information plane 1 is aphase structure comprising a partially reflecting layer provided with arelief between two transparent layers of equal refractive indices. Suchan information plane generates diffraction beams in reflection only. Anincident radiation beam 33 with an amplified a reflected from theinformation plane 1 not only yields a zero-order diffraction beam 34with an amplitude a√R₁ in reflection, but also higher-order diffractionbeams with an amplitude a√R₁ m₁. If the information plane 2 were notpresent, these zero-order and higher-order beams would interfere on thedetector and yield an interference signal in the detector signal whichis proportional to a² m₁ R_(l). The factor m₁ is thus the modulationfactor of the detection signal generated by the information plane 1 inthe absence of other information planes. Due to the presence of theinformation plane 2, the zero-order beam 34 of the information plane 1is intensified with the beam 35. The sum of the beams 34 and 35 nowinterferes on the detector with the higher-order diffraction beamsformed by the information plane 1. This yields the followinginterference signal at the output of the detector: ##EQU11## With acomparable frequency content of the two information planes and the samemodulation transfer function, the interference requirement (2a) will be:##EQU12## At customary values of R₁, T₁, and R₂ of 0.1, 0.8 and 0.4,respectively, the modulation factors should now comply with ##EQU13##The modulation factor m₁ can be rendered small by choosing thedouble-pass phase depth of the phase structure in the information plane1 for the wavelength λ₂ to be approximately equal to 2 π, as is apparentfrom said book "Principles of Optical Disc Systems". However, the phasedepth should be such that there is a large modulation factor on readingthe information plane 1 with radiation of the wavelength λ₁ requiringthe phase depth to be different from 2 π. The ratio between themodulation factors in equation (9) can be rendered more favourable byincreasing the reflection R₂. To be able to read the information plane 1also satisfactorily, R₁ and R₂ are preferably wavelength-dependent dueto the choice of the material of the information planes. At thewavelength λ₁, R₁ should then be large and R₂ should be small, and viceversa at λ₂. If the interference requirement is difficult to meet withthe available materials for the information planes, the interferencesignal of the information plane 1 may be further reduced by increasing,for example the distance between the two information planes so that themodulation transfer function of the interference planes will be smaller.

A reasoning analogous to the foregoing may be given for reading theinformation plane 1 with radiation of the wavelength λ₁. In theembodiment of the record carrier shown in FIG. 3D, the information plane1 does not form any higherorder diffraction beams in transmission. Inrecord carriers which do form such beams, the expressions (6) to (9)should be adapted.

The examples mentioned hereinbefore indicate how the parameters of therecord carrier must be determined in simple cases so as to comply withthe interference requirement. In more complicated cases, particularlythose in which the intermediate layer 8 has a thickness of the order ofthe wavelength, the computation may be performed in an analogous manner,in which phase jumps at optical transitions, phase rotations in thelayers and multiple reflections between the transitions must be takeninto account in known manner. However, this computation will then belargely numerical and will not have the formulary simplicity shownhereinbefore. The above arguments can also be applied to record carriersin which marks generate higher harmonics of the radiation used forreading the marks.

A second possibility of reducing the interference signals is to make useof a reading device with different detection modes for the differentinformation planes provided with different information structures. Anexample is an information storage system in which the information in therecord carrier is stored in the form of a magnetic domain structure aswell as a pit structure, each being read in a specific detection modeadapted to the structure. Such an information storage system comprisinga record carrier 5 and a reading device 6 is shown in FIG. 4A. Aradiation source 11 in the reading device supplies a radiation beamwhich is passed via a, for example partially polarizing, beam splitter25 to an objective system 14 and focused by this system on aninformation plane of the record carrier 5. The beam reflected by therecord carrier is incident via the beam splitter 25 on a beam splitter36 which splits the beam into two sub-beams having mutuallyperpendicular directions of polarization. The two sub-beams are incidenton separate detectors 37 and 38 whose outputs are connected to a circuitcomprising a differential amplifier 39 and a summing amplifier 43. Thetwo detectors each convert the radiation of the incident sub-beam intoan electric signal. The differential amplifier 39 forms a detectionsignal 40 which is the difference of the two electric signals. Thesumming amplifier 43 forms a detection signal 44 which is the sum of thetwo electric signals. Each of the detection signals 40 and 44 isconverted by detection circuits 41 and 45 into information signals 42and 46, respectively. The components 37, 38, 39 and 43 jointlyconstitute a detection system.

FIG. 4B shows a part of the record carrier 5 in a cross-section. Therecord carrier has two information planes, an information plane 1 inwhich the information is stored in pits formed in the subjacentsubstrate 7 and an information plane 2 in which the information isstored in magnetic domains formed in a magnetizable layer. Theinformation planes may be separated from each other by an intermediatelayer 8, as is shown in the Figure, but they may also coincide. If thescanning beam 32 is focused on the information plane 1, as is shown inthe Figure, the beam will be modulated by the pits. The resultant powermodulation of the reflected beam is detected in accordance with a firstdetection mode, i.e. electric signals supplied by the detectors 37 and38 are added by the summing amplifier 43, at which a pit detectionsignal 44 is obtained. If the scanning beam 32 is focused on theinformation plane 2, the beam will be modulated due to the changingstates of magnetization. The resultant polarization modulation of thereflected beam is detected in accordance with a second detection mode,i.e. the electric signals supplied by the detectors 37 and 38 aresubtracted from each other by the differential amplifier 39, at which adomain detection signal 40 is obtained.

For an information storage system in which the two information planescoincide, the interference requirement for reading the domains followsfrom equation (2a) with E₁ =E₂. The modulation transfer functions MTF inthe interference requirement must be determined in the second detectionmode, i.e. for the transmission of the modulations in the radiation fromthe record carrier to the detection signal 40. The transfer function forthe modulation generated by the pit structure in the information planeis in this case equal to the product of an attenuation factor Z and thetransfer function for the modulation generated by the pit structure. Thefactor Z is the ratio of the modulation in the domain detection signal40 due to pits which have unwantedly been read and the modulation in thepit detection signal 44 due to the same pits. The pits generate electricsignals in the detectors 37 and 38 with principally have the same shape,and the domain detection signal 40 supplied by the differentialamplifier 39 should be zero. The fact that the domain detection signal40 is not equal to zero is caused by small deviations in the recordcarrier 5, in the optical path from the record carrier to the detectors37 and 38 and in the electronic circuit in the detection system. Adomain detection signal 40 generated by the pits will, however, beconsiderably weaker than the pit detection signal 44 generated by thepits at the output of the summing amplifier 43. If the informationcontents of the pits and the domains are comparable and both types ofmarks, if read in their own detection mode, generate an equally strongread signal in their own detection mode, the interference requirementcan be written as

    Z<Q.                                                       (10)

Q is the interference ratio for the detection circuit 41. Equation (10)supplies the requirement for the record carrier and the optical systemof the reading device in order to suppress the pit information in thesecond detection mode for the domains. If this requirement is satisfied,the influence of the pits will be sufficiently small when reading thedomains in the second detection mode.

The interference requirement (2a) for reading the information plane 2 inFIG. 4B in an information storage system in which the information planesin the record carrier do not coincide is ##EQU14## The power in thebeams coming from the two information planes is determined by thereflection and transmission coefficients of the two planes. If thethickness d of the intermediate layer 8 between the two informationplanes is larger than the depth of field of the objective system 14, theradiation reflected by the information plane 2 will not have a largeinfluence on the interference signal generated by the information plane1 when this information plane 1 is being read, which is in contrast tothe record carrier shown in FIG. 3C in which this influence makes itselffelt due to the small distance between the information planes. The powerin the radiation reflected by the record carrier and received by thedetection system is then equal to the sum of the powers of theradiations reflected by the separate information planes. The ratio ofthe power in the radiation reflected by the two information planes isthus given by ##EQU15## The transfer function MTF(f,0) of the modulationgenerated by the information plane 2 is equal to the known modulationtransfer function for an incoherently illuminated system. The transferfunction for the modulation generated by the interference plane 1 is adefocused transfer function of the modulation generated by theinformation plane 1 located outside the focus and multiplied by theabove-mentioned attenuation factor Z.

The interference requirement (11) can now be written as ##EQU16## inwhich the index 1 of MTF₁ indicates that the modulation transferfunction must be determined in the first detection mode, while the useof the second detection mode for reading the information plane 2 istaken into account with the factor Z. At a given configuration andparameters of the record carrier, the requirements can now be imposed onthe reading device as regards the interference ratio Q and theseparation between the detection signals 40 and 44, expressed as arequirement for Z. On the other hand, at given values of Q and Z, theparameters of the record carrier can be determined. In an informationstorage system having such a thickness of the intermediate layer 8 thatthe ratio of the modulation transfer function MTF₁ and MTF is equal to0.15, and for which it further holds that: Q=0.03, Z=0.033, m₁ =m₂, R₂=0.3 and T₁ =1-R₁ (i.e. a negligible radiation absorption in theinformation plane 1), the reflection coefficient R₁ of the informationplane 1 should be smaller than 0.48 in order to comply with theinterference requirement (13). Starting from given values of Q, Z, R₁,R₂, m₁ and m₂, the maximum ratio for the transfer functions MTF₁, andMTF can also be determined by means of equation (13). The minimumdistance between the information planes at which the information canstill be read satisfactorily follows therefrom. When the minimumdistance is used, the record carrier 5 has the largest informationdensity.

In a comparable manner the interference requirement can be given forreading the information plane 1 with the pit structure in the firstdetection mode. The pit detection signal 44 generated by the informationplane 1 has a DC value and an AC value. It is therefore generallydetected in the detection circuit 45 in the way as described withreference to FIG. 2. The detection level D is then at the DC value ofthe detection signal. The presence of radiation coming from theinformation plane 2 in the record carrier not only leads to an unwantedmodulation of the pit detection signal 44, but also to an offset of theDC value. This offset may in turn lead to an unreliable detection of themodulation generated by the information plane to be read. The allowedoffset, at which detection can still be performed reliably, can beexpressed in a DC interference requirement. The AC interferencerequirement then provides the allowed AC interference signal of theinformation plane 2. The offset may be expressed as a fraction of theamplitude of the modulation of the pit detection signal 44 or as afraction of the DC value. In the latter case the DC interferencerequirement is given by equation (3). If the powers of the beams satisfythe equation (12), the DC interference requirement can be written as##EQU17## The modulation transfer function indicates how much radiationof the information plane 2 which is out of focus, is received by thedetection system. This quantity can be reduced considerably byperforming a detection known as confocal detection. For an informationstorage system having such a thickness d of the intermediate layer 8 andsuch a configuration of the detection system that MTF₁ =0.15 and the DCinterference ratio Q=0.05, R₁ must be larger than 0.36 if R₂ =0.3. Incombination with the interference requirement computed hereinbefore forreading the information plane 2, it follows that the intensityreflection coefficient of the information plane 1 should have a value ofbetween 0.36 and 0.48.

FIG. 5A shows a record carrier 5 for reading the information inreflection, in which the interference requirement leads to specialrequirements to be imposed on the parameters of the record carrier. Therecord carrier has three information planes, numbered 1, 2, 3 from theside where the radiation beam 32 from the objective system (not shown)enters the record carrier. Two intermediate layers having thicknesses d₁and d₂ are provided between the information planes 1 and 2, and 2 and 3,respectively. The Figure shows the situation where the information plane3 is read by means of the radiation beam 32. The radiation reflected bythe information plane 2 forms a converging beam 50, denoted by brokenlines, in the direction of the information plane 1. If the intermediatelayers have the same thickness, this converging beam will focus on theinformation plane 1. A part of the radiation in the converging beam willbe transmitted by the information plane 1 and form a diverging beam 51which is received by the objective system. Another part will also returnto the objective system after further reflections from the informationplanes 1 and 2. Consequently, a part of the radiation beam returning inthe objective system is focused on the information plane 3 to be readand simultaneously another part is focused on the interference plane 1.The radiation of beam 50 reflected from the information planes 1 and 2will thus generate a large interference signal. The beam 51, transmittedby the information plane 1, is far out of focus for the objective systemand will generally give a much smaller interference signal than the beamreflected from the information plane 1. The interference requirement(2a) for reading information plane 3 is then: ##EQU18## in which thefirst term above the bar of division is the interference signalgenerated by the information plane 2 and the second term is theinterference signal generated by the information plane 1. The secondterm can be considered as the interference signal generated by animaginary information plane formed by mirroring of the information plane1 with respect to the information plane 2. The imaginary informationplane has the distance (d₁ -d₂) to the information plane 3 to be readand an effective reflection coefficient R₁ (T₁ R₂)².

If d₁ =d₂, as shown in FIG. 5A, the imaginary information planecoincides with the information plane 3. The modulation transfer functionin the second term of equation (15) then has the focused value and isthus relatively large. In such a case the first term in equation (15)can generally be ignored. If the information or frequency contents andthe modulation factors of the information planes 1 and 3 are comparable,the interference requirement (15) can be written as ##EQU19## At equalvalues of the reflection coefficients and a negligible absorption in theinformation planes, the reflection coefficient should be smaller than0.15 so as to satisfy an interference requirement with Q=0.03. In arecord carrier having equal distances between the information planes,the information planes should thus not have too high a reflectioncoefficient. However, the information plane which is furthest remotefrom the objective system, i.e. the information plane 3 in FIG. 5A, mayhave a high reflection coefficient.

A record carrier with three information planes must comply with threeinterference requirements, one for reading each of the planes, therebyreducing the design ranges for the parameters of the record carrier. Thedesigner of the record carrier shown in FIG. 5A has more freedom if hegives the intermediate layers 8 and 9 different instead of equalthicknesses. This results in a decrease of the value of the modulationtransfer function in the second term of equation (15) so that thestrength of the interference signal generated by the information plane 1is reduced. A large degree of freedom in the choice of the values of theparameters of the record carrier is obtained when the ratio of theoptical thicknesses i.e. d/n of the intermediate layers is larger than1.5. In order to maintain a high information density, the ratio shouldbe smaller than 3, and, preferably, smaller than 2. The lowestinterference signals are obtained when the thinner intermediate layer iscloser to the entrance face of the record carrier through which theradiation beam 32 enters the record carrier than the thickerintermediate layer.

In a record carrier having more than three information planes such asthe record carrier depicted in FIG. 3, attention should be paid duringdesign to the positions of all imaginary information planes resultingfrom reflections. If an imaginary information plane coincides, after twoor more specular reflections on other information planes, with anexisting information plane, this will generally not lead to inadmissiblylarge interference signals because the strength of the interferencesignals rapidly decreases at an increasing number of reflections of theradiation on information planes. A multiplane record carrier cantherefore often be realised satisfactorily by alternately placing athicker and a thinner intermediate layer between the information planes,as shown in FIG. 5B. FIG. 5B depicts at least six (6) informationplanes. These planes are denoted with reference numerals 1,2,3,97,98 and99. Furthermore, at least three intermediate layers 8,9 and 8A aredepicted. The thickness of a single intermediate layer is thendetermined by the interference requirement for the two informationplanes at both sides of the intermediate layer. The difference inthickness between two consecutive intermediate layers follows from theinterference requirement (15). The record carrier shown in FIG. 5B caneasily be made by stacking thin transparent sheets of equal thickness,with an information plane on each side of a sheet, and with a spacer ofair or a transparent material in between the sheets.

The quantity of radiation detected from an information plane read inreflection is dependent, inter alia on the transmission of theinformation planes lying between the information plane to be read andthe objective system. Generally, the power of the read signal generatedby the information plane to be read will therefore decrease with anincreasing distance of this information plane to the objective system.However, for a multiplane record carrier the power of the read signal ofthe different planes j, in which the value of j increases with anincreasing distance to the objective system, can be maintained at aconstant value if the condition ##EQU20## is satisfied. For a four-planerecord carrier with an ideal reflector for the fourth information planeand without absorption in the information planes, this leads to R₁=0.16, R₂ =0.23, R₃ =0.38 and R₄ =1.00.

The modulation transfer function MTF occurring in the interferencerequirement is dependent on the optical parameters of the readingdevice. Before giving an expression for the MTF, the parametersoccurring therein will be dealt with first. The MTF is dependent, interalia on the wavelength λ of the radiation with which the record carrieris read and on the numerical aperture (NA) of the objective system 14 atthe side of the record carrier. These parameters determine the highestfrequency which is still transmitted by the objective system, i.e. thehighest spatial frequency of the marks which can still be readseparately. The highest transmitted frequency for a normal coherentlyilluminated optical system, referred to as the cut-off frequency, isgiven in terms of spatial frequencies by ##EQU21## or, in terms oftemporal frequencies, by ##EQU22## in which v is the velocity at whichthe spot scans the information plane. The highest transmitted frequencyfor the reading device is equal to 2f_(c). The frequencies present inthe information stored in the information planes and in the detectionsignals can be represented by the dimensionless parameter ω , with##EQU23## Likewise, the distance d between the information planes can begiven in terms of the depth of field of the object system ##EQU24## Whendetermining the interference requirement, the distance between theinformation plane to be read and an interference plane is equal to thedefocusing. Hence, the parameter ξ is a measure of the defocusing of aninformation plane with respect to the spot with which the information inthe record carrier is read. The dependence of the modulation transferfunction on the size of the detection system 15 can be described bymeans of the parameter η, given by ##EQU25## Here, r_(d) is theeffective radius of the detection system. If no diaphragms are arrangedin the radiation beam in front of the detection system, the effectiveradius is equal to the radius of the radiation-sensitive surface of thedetection system. If a diaphragm has been arranged, for example at apoint of convergence of the radiation beam just in front of thedetection system, as is common practice for confocal detection, theeffective radius is equal to the radius of the diaphragm. The parameterM in equation (22) is the magnification of the optical system from therecord carrier 5 to the detection system 15. The optical system in thedevice shown in FIG. 1 comprises the objective system 14 and thecollimator lens 13.

The information in the information planes will often be ordered intracks. The dependence of the modulation transfer function on the periodq of the tracks is expressed in the dimensionless parameter κ as follows##EQU26## The direction parallel to the tracks and in the informationplane is denoted by the subscript t of tangential in the followingequations. The direction perpendicular to the tracks and in theinformation plane is denoted by the subscript r of radial.

The modulation transfer function for a record carrier read in reflectioncan be written in the following way as the product of a tangentialmodulation transfer function MTF_(t) and a radial function F_(r)

    MTF(f,d)=MTF.sub.t (f,d)F.sub.r (q,d).                     (24)

The tangential modulation transfer function is given in dimensionlessparameters by ##EQU27## The parameter d should be replaced by d/n if therefractive index of the intermediate layers of the record carrier is notequal to 1. Dependent on the size of the detection system, theintegration area S is equal to ##EQU28## if ##EQU29## if ##EQU30## and(28) the area of overlap of said two areas if ##EQU31## In the case of alarge radiation-sensitive surface of the detection system theintegration is over the area of overlap of the zero-order andhigher-order diffraction beams incident on the detection system(integration area given by equation (26)); in the case of a smallradiation-sensitive surface the integration within the area of overlapis only over the surface of the detection system (integration area givenby equation (27)). It is apparent from equations (26), (27) and (28)that the parameter ω, i.e. the frequency contents of the informationplane, also determines whether the radiation-sensitive surface isregarded as large or small. If the sensitive surface is smaller than theimage of the spot on the detection system, there is confocal detection.In dimensionless parameters this corresponds to η<1/(2ξ). In the case ofconfocal detection the value of the modulation transfer function veryrapidly decreases with an increasing defocusing ξ. Information planesnot to be read then produce only small interference signals.Consequently, the distance between the information planes can be furtherreduced than is possible without confocal detection, which enhances theinformation density of the record carrier. The tangential modulationtransfer function for a large detection system is equal to themodulation transfer function of an incoherently illuminated imagingsystem as is known, inter alia from the article "The defocused image ofsinusoidal gratings" by W. H. Steel in Optica Acta, vol. 3, no. 2, June1956, pp. 65-74.

The radial function F_(r) (q,d) in equation (24) is given indimensionless parameters by ##EQU32## if ξ<2,1 and ##EQU33## if ξ≧2,1.The function sinc(x) is defined as sin(x)/x. The radial factor F_(r) isbased on the fact that the information in the track to be read and theinformation in adjacent tracks are not correlated in phase. Themodulation transfer function is normalized at zero frequency and zerodefocusing for an infinitely large detector. Theoretically, themodulation transfer function locally has deep and sharp minimum valuesas a function of the defocusing ξ. However, these minimum values onlyoccur with information planes in which only a single pure frequencyoccurs. In most cases the information will have a spectrum offrequencies so that the minimum values will not occur in the modulationtransfer function. For determining the modulation transfer function theenvelope of the function defined in equation (24) will then preferablybe taken.

The envelope of the above-mentioned equation (24) for the modulationtransfer function is in good conformity with the measured transferfunction of an information storage system. FIG. 6 shows by way ofexample the modulation transfer function as a function of thedimensionless defocusing ξ for the parameter values ω=0.6, κ=1 and ηlarge. The curve is normalized at zero defocusing, i.e. 20 ¹⁰ log|MTF(0.6,ξ)|/MTF(0,ξ)! is plotted along the vertical axis. The solidline indicates the value of equation (24), the broken line indicates theenvelope. The value U of the envelope for ξ>2.5, again normalized atzero defocusing, is given in a good approximation by ##EQU34##

It will now be described by way of example how the distance between theinformation planes required for a record carrier according to theinvention can be derived with the aid of FIG. 6 which results from theinventive idea and from the insight on which this invention is based. Aninformation storage system has a detection circuit with an interferenceratio for interference signals generated by information planes which arenot to be read currently of Q=0.01=-40 dB. The record carrier has twoinformation planes with information coded in marks in the same way, withcomparable frequency spectra and complying with equation (17). Thelowest frequency in the spectrum of the information plane which iscurrently not read produces the largest crosstalk, because generally thevalue of the modulation transfer function decreases with increasingfrequency. To attain a reasonable approximation of the interferencesignal, it is assumed that only the lowest frequency occurs in thespectrum. If this is a frequency with ω=0.6, FIG. 6 will give theassociated modulation transfer function. The interference requirementamounts to finding the smallest value of ξ for whichMTF(0.6,ξ)/MTF(0.6,0)<Q. The broken line in FIG. 6 of the envelope givesξ=5.2 for -40 dB. It follows from equation (21), at a wavelength λ=0.8μm and a numerical aperture NA=0.5 that d=8.4 μm. If various frequenciesoccur in a band of the spectrum, a weighted average of the MTF of thisband should be determined. The minimum thickness of the intermediatelayer between the two information planes applies to the case where theintermediate layer is air. If the intermediate layer has a refractiveindex of more than 1, for example n=1.5, d/n must be 8.4 μm, hence dmust be 12.6 μm. Said thicknesses are the minimum thickness of theintermediate layer in which the information in the information planescan still be read satisfactorily, or in other words, the detectioncircuit 17 in the reading device can derive a reliable informationsignal S_(i) from the detection signal S_(d). At the minimum thicknessthe information density in the direction perpendicular to theinformation planes in the record carrier of the information storagesystem is maximal. If the thickness of the intermediate layer is largerthan this value and one of the information planes is being read, theother information plane will cause an interference signal in thedetection signal S_(d), which interference signal is more than 40 dBbelow the level of the read signal.

The modulation transfer function for the zero frequency as used, forexample for the DC interference requirement in equation (14) is given by

    MTF(0,d)=1 if η≧1,                              (32)

and

    MTF(0,d)=η.sup.2 if η<1.                           (33)

A value MTF=0.15 as used in the consideration following equation (14)can be realised with η=0.387. It follows from equation (22) that at aradius r_(d) =20 μm, a magnification M=10 and a numerical apertureNA=0.5 the defocusing or the thickness of the intermediate layer 8should be 5.2 μm or more in this case.

The modulation transfer function for reading devices with a non-uniformintensity distribution over the radiation beam proximate to theobjective system 14 and with optical aberrations can be determined bymeans of numerical methods as described, for example in said book"Principles of Optical Disc Systems" and in the book "Scanning OpticalMicroscopy" by T. Wilson and C. Sheppard (Academic Press, 1984) chapters2, 3 and 4, and can subsequently be further used in (24) in theinterference requirement (2a) or (2b).

The embodiments, hitherto described, of the information storage systemaccording to the invention are adapted to detect radiation reflected bythe record carrier. However, in principle, all above-described methodsof satisfying the interference requirement are also applicable toinformation storage systems in which the radiation is detected which istransmitted by the record carrier. The consequences of the interferencerequirement for an information storage system operating in transmissionare largely equal to those for an information storage system operatingin reflection. FIG. 7A shows diagrammatically an information storagesystem operating in transmission. In the reading device 6' a radiationsource 11 generates a radiation beam which is focused on an informationplane of the record carrier 5' by a collimator lens 53 and an objectivesystem 14. The radiation transmitted by the record carrier iscollimated, for example by a collector lens 54 and focused on adetection system 15 by a further collimator lens 55. The detectionsystem supplies a detection signal S_(d) which is converted into aninformation signal S_(i) by a detection circuit 17. For a correctoperation of the reading device, the objective system 14 and thecollector lens 54 should be centred on one and the same optical axisduring reading, so that the components 11, 53, 14 on the one hand andthe components 54, 55 and 15 on the other hand are to be moved by acommon driving device for displacing the radiation beam in the radialdirection on a round record carrier 5'.

FIG. 7B shows a cross-section of part of the record carrier and theradiation beam 32 focused on the information plane 1 of the recordcarrier. The power of the transmitted beam 56 is independent of theinformation plane on which the beam is focused. Hence, the power E ofthe radiation beam coming from an information plane is equal in theinterference requirement (2a) or (2b) for all information planes, or

    E.sub.j =E.sub.i for all j and i.                          (34)

The read signal or the interference signal in the detection signal S_(d)generated by an information plane, is thus only dependent on themodulation generated by the relevant information plane in thetransmitted radiation and is not dependent on the possibly differenttransmissions of the information planes or of the ordinal number ofthese planes. The interference requirement (2a) for reading theinformation plane i can be written as ##EQU35## The interferencerequirement (2b) can be rewritten in the same way.

Likewise as the modulation transfer function of an information planeread in reflection, the modulation transfer function MTF^(l) for aninformation plane read in transmission can be written as a product of atangential modulation transfer function MTF_(t) ^(l) and a radialfunction F_(r) :

    MTF.sup.l (f,d)=MTF.sub.t.sup.l (f,d)F.sub.r (q,d).        (36)

The radial function is given in equations (29) and (30). In principle,the tangential modulation transfer function is known from said book"Scanning Optical Microscopy". If the numerical apertures of theobjective system 14 and the collector lens 54 are equal, and if theoptical aberrations are small, the tangential modulation transferfunction can be written in dimensionless parameters as: ##EQU36## J₁ thefirst-order Bessel function and S being the area defined by ##EQU37##and -ζ<ε<ζ. The parameters ω and ξ are defined in equations (20) and(21), respectively. The parameter ζ is given by: ##EQU38## and is thedimensionless radius of the image which is formed of theradiation-sensitive surface of the detection system 15 by means of theoptical system with the components 55 and 54. The parameter M is themagnification of the optical system from the record carrier 5' to thedetection system 15. The modulation transfer function is normalized atzero frequency and zero defocusing for an infinitely large detector.

The same methods as described for the record carrier read in reflectioncan be used to satisfy the interference requirement (35). An advantageof the record carrier suitable for reading in transmission is that themodulation transfer function as a function of the defocusing ξ decreasesmore rapidly in value than the modulation transfer function for recordcarriers suitable for reading in reflection. Consequently, theinformation planes in the last-mentioned record carrier may be situatedcloser together than in the record carrier suitable for reading inreflection. A further advantage of reading in transmission is thatimaginary information planes caused by mirroring on other informationplanes do not have much effect on the strength of the interferencesignals.

We claim:
 1. An optical record carrier comprising at least twoinformation planes for storing information which can be read by areading device which scans the record carrier from one sidethereof;which device includes an optical system for focusing a radiationbeam on one of the planes so that that plane can be read, a detectionsystem for receiving radiation from the record carrier resulting fromthe radiation beam and producing a detection signal based thereon, whichdetection signal includes one or more interference signals resultingfrom the one or more planes which are not being read and a read signalresulting from the plane which is being read, and a circuit for derivingan information signal, corresponding to the information included in theplane being read, from the detection signal, which circuit has aninterference ratio Q associated therewith indicative of an operationalcharacteristic of the circuit; wherein the planes are at a distance fromone another and have optical properties such that a ratio of a sum ofthe one or more interference signals and the read signal is smaller thanthe interference ratio Q.
 2. The optical record carrier as claimed inclaim 1, comprising at least three information planes includinginformation having a substantially equal frequency spectrum, theinformation planes having substantially equal modulation factors andbeing spaced at substantially equal distances, wherein ##EQU39## inwhich R₁, R₂ and R₃ are the radiation intensity reflections coefficientsof information planes 1, 2 and 3, respectively, and T₂ is the radiationintensity transmission coefficient of information plane
 2. 3. Theoptical record carrier as claimed in claim 1, wherein the at least twoinformation planes are successively an increasing distance from thedetection system and have a radiation intensity reflection coefficientsR_(j) which substantially comply with the formula ##EQU40## where T_(j)is the radiation intensity transmission coefficient of information planej.
 4. The optical record carrier as claimed in claim 1, including atleast three information planes and two intermediate layers separatingthe information layers from one another, the radiation beam having adepth of field and in which the difference between d₁ /n₁ and d₂ /n₂ isat least three times the depth of field of the radiation beam, in whichd₁, d₂ and n₁, n₂ are the thicknesses and refractive indices of thefirst and second intermediate layers, respectively, and the depth offield is λ/(2 NA²), where λ and NA are the wavelength and numericalaperture of the radiation beam, respectively.
 5. The optical recordcarrier, as claimed in claim 1, including at least three informationplanes and two intermediate layers separating the information layersfrom one another, in which the difference between d₁ /n₁ and d₂ /n₂ isat least 4 μm, in which d₁, d₂ and n₁, n₂ are the thicknesses andrefractive indices of the first and second intermediate layersrespectively.
 6. The optical record carrier as claimed in claim 1,including at least three information planes and two intermediate layersseparating the information planes from one another, the ratio of theoptical thicknesses of the intermediate layers ranging between 1.5 and3.
 7. The optical record carrier as claimed 1, including at least fourinformation planes and at least a first, a second and a thirdintermediate layer, which intermediate layers separate the informationplanes from one another, wherein the first and third intermediate layersare of essentially equal thicknesses, the second intermediate layer hasa thickness which is not equal to the thickness of the first and thirdintermediate layers, and the second intermediate layer is disposedintermediate the first and third intermediate layers.
 8. The opticalrecord carrier as claimed in claim 1, including at least threeinformation planes which are at a distance from one another and haveoptical properties such that the number of interference signalsresulting from at least two of the information planes not being readwhich make up the sum of interference signals is at least two.
 9. Therecord carrier as claimed in claim 1, wherein the interference ratio Qis indicative of a limit value for the sum of the one or moreinterference signals below which the detection circuit can reliablyderive the information signal from the detection signal.